The present invention relates to a process for the production of a synthetic natural gas, more particularly, to the production of synthetic natural gas from a secondary synthesis gas, by which term is meant a synthesis gas produced from the coal derived liquid hydrocarbon by-product resulting from the gasification of coal to produce gasifier synthetic natural gas. The secondary synthesis gas is methanated to provide additional synthetic natural gas which may be blended with the gasifier synthetic natural gas.
Coal gasification technology is well known that has been in commercial use in South Africa since about 1954 and was commonly used in the United States prior to 1950 to make town gas. The most commonly employed gasifier process is that developed by Lurgi Kohle und Mineraloeltechnik GmbH, Frankfurt (Main), Federal Republic of Germany. The Lurgi process utilizes a fixed bed gasifier in which coal of a selected particle size is fed into the top of the gasifier countercurrently to a stream of steam and oxygen fed from the botton of the gasifier. A synthesis gas (herein and in the claims called gasifier synthesis gas) and a hydrocarbon liquid by-product are produced from the coal and withdrawn from the top of the gasifier. Solid ash residue is withdrawn through a rotating grate at the bottom of the gasifier. Up to about one fourth of the coal fed to the process will emerge as the liquid hydrocarbon by-product, rather than as the desired gasifier synthesis gas. Thus, a gasification plant using 8 million tons of coal per year may produce as much as about 2 million tons of liquid hydrocarbon by-product.
Such liquid hydrocarbon by-product essentially comprise three major fractions classified as oil, tar and phenolics. The oil fraction has a boiling range of about 200.degree.-600.degree. F. (93.degree.-316.degree. C.) and requires treatment if it is to be employed as a petroleum product substitute. The tar contains substantial quantities of oxygen and nitrogen and about 0.01 percent ash. The phenolics are somewhat similar to cresols and have a boiling range of about 290.degree.-400.degree. F. (143.degree.-205.degree. C.). Generally, these by-products are not particularly valuable and do not command a high price even in those areas where a market exists for them. The liquid hydrocarbon by-product is also carcinogenic and toxic. Disposition of the by-product when no market exists for it presents significant environmental and economic problems.
The present invention enables the conversion of such liquid hydrocarbon by-product into additional synthesis gas (herein and in the claims called secondary synthesis gas). This "secondary synthesis gas" (sometimes herein abbreviated to "secondary SG") is to be distinguished from the "gasifier synthesis gas" (sometimes herein abbreviated to "gasifier SG") which is obtained in the coal gasification step. Methanation of the gasifier SG and secondary SG is carried out to provide product synthetic natural gas (sometimes herein abbreviated to "SNG"). As explained below, the conversion of the liquid hydrocarbon by-product to secondary SG is carried out by a catalytic partial oxidation process in which steam reforming and hydrocracking reactions are believed to also take place, and to provide an efficient and economical means of converting the liquid hydrocarbon by-product.
Steam reforming is a well known method for treating hydrocarbons to generate hydrogen therefrom. It is usually carried out by supplying heat to a mixture of steam and a hydrocarbon feed while contacting the mixture with a suitable catalyst, usually nickel. Steam reforming is generally limited to paraffinic naphtha and lighter feeds which have been de-sulfurized and treated to remove nitrogen compounds, because of difficulties in attempting to steam reform heavier hydrocarbons and the poisoning of steam reforming catalysts by sulfur and nitrogen compounds.
Another known method of obtaining hydrogen from a hydrocarbon feed is partial oxidation, in which the hydrocarbon feed is introduced into an oxidation zone maintained in a fuel rich mode so that only a portion of the feed is oxidized.
It is known that steam may also be injected into the partial oxidation reactor vessel to react with the feed and with products of the partial oxidation reaction. The process is not catalytic and requires high temperature to carry the reactions to completion, resulting in a relatively high oxygen consumption. On the other hand; the partial oxidation process has the advantage that it is able to readily handle hydrocarbon liquids heavier than paraffinic naphthas and can even utilize coal as the source of the hydrocarbon feed.
Catalytic autothermal reforming of hydrocarbon liquids is also known in the art, as evidenced by a paper Catalytic Autothermal Reforming of Hydrocarbon Liquids by Maria Flytzani-Stephanopoulos and Gerald E. Voecks, presented at the American Institute of Chemical Engineers' 90th National Meeting, Houston, Tex., Apr. 5-9, 1981. Autothermal reforming is defined therein as the utilization of catalytic parital oxidation in the presence of added steam, which is said to increase the hydrogen yield because of simultaneous (with the catalytic partial oxidation) steam reforming being attained. The paper discloses utilization of a particular bed of three different nickel catalysts into which steam, air and a hydrocarbon fuel supply comprising a No. 2 fuel oil are injected. The resulting product gases contain hydrogen and carbon oxides.
In Brennstoff-Chemie 46, No. 4, p. 23 (1965), a German publication, Von P. Schmulder describes a Badische Anilin and Soda Fabrik (BASF) process for autothermal reforming of gasoline. The process utilizes a first, pelletized, i.e., particulate, platinum catalyst zone followed by a second, pelletized nickel catalyst zone. A portion of the product gas is recycled to the process.
Disclosure of the utilization of a noble metal catalyzed monolith to carry out catalytic partial oxidation to convert more than half of the hydrocarbon feed stock upstream of a steam reforming zone is disclosed in an abstract entitled Evaluation of Steam Reforming Catalyst for use in the Auto-Thermal Reforming of Hydrocarbon Feed Stocks by R. M. Yarrington, I. R. Feins, and H. S. Hwang (National Fuel Cell Seminar, July 14-16, 1980, San Diego). The abstract noted the unique ability of rhodium to steam reform light olefins with little coke formation and noted that results were obtained for a series of platinum-rhodium catalysts with various ratios of platinum to total metal in which the total metal content was held constant.
U.S. Pat. No. 4,054,407, assigned to the assignee of this application, discloses two-stage catalytic oxidation using platinum group metal catalytic components dispersed on a monolithic body. At least the stoichiometric amount of air is supplied over the two stages and steam is not employed.
U.S. Pat. No. 3,481,722, assigned to the assignee of this application, discloses a two stage process for steam reforming normally liquid hydrocarbons using a platinum group metal catalyst in the first stage. Steam and hydrogen, the latter of which may be obtained by partially cracking the hydrocarbon feed, are combined with the feed to the process.